When towing with an EV, that precious range value that so many are fixated on - usually plummets - often down to 50% of the original value without a tow load. There are many factors that will affect this value.
Some of this range loss is due to the extra weight of the combined vehicles, however the extra drag that towing introduces is also a significant factor. Any extra wind resistance a trailer or caravan (or roof rack etc) adds to the equation, means the tow vehicle is required to exert extra energy in order to maintain a given speed.
The theory I am testing is that if we can minimise the aerodynamic drag of the caravan/camper trailer behind our EV - we should be able to reduce the range loss we would otherwise experience. This equates to either A) savings in recharge("/fuel") costs and/or B) increased range between charges. How much gain we can achieve is yet to be determined - but our main target is to achieve the current maximum distance between charging options"- which is approximately 270kms. Generally, heading east-west across the north WA-NT-QLD and across the Nullarbor are the most difficult sections.
There are currently limited options for tow vehicles in Australia. Among the contenders were the Kia EV9, Tesla Model Y Long Range and the Volvo XC40 Twin Recharge. All have an (unladen) range of approximately 500kms - plus and minus some pros and cons of each when comparing things like space and tow weight limits etc. If you take the rough estimate of ~50% range drop when towing an average load it is evident that the range then drops to under the 270kms needed to circumnavigate Australia with our current EV charging infrastructure. (Although this is obviously changing weekly!)
Therefore, the plan was to make aerodynamic improvements to ensure the best chance of exceeding the 270km range when towing from a full charge. (Especially because there will always be other factors such as headwinds, temperatures, tyre pressures, elevation changes etc etc that can go in or against your favour.)
The basic premise I have been working on is:
Remove / minimise the secondary impact of airflow over the car, hitting a blunt wall of the front of the trailer. Car front ends are (or should be) designed to penetrate the air as gently as possible as this frontal area is the biggest energy loss/drag. If clean air leaving the rear of the car hits another "front", then all that loss gets doubled.
Minimise the drag of air flow leaving the towing combination. Similarly, car rear ends are designed to smooth airflow behind and minimise "suction" of low pressure areas behind the vehicle. I like the analogy of running your hand through a bath of water "flat" vs "sideways". When your fingers are pointing in the direction of travel, your hand glides effortlessly through the water. When you turn your hand sideways, there is a lot of resistance needed to push through the water and behind your hands are turbulent waters of mini whirlpools and eddy currents. The aerofoil shape of a wing is tapered at the back (trailing edge) to allow airflow from over the top, and from underneath to rejoin smoothly which minimises trailing drag. I planned to create that same aerofoil trailing edge, instead of the usual blunt rear end of a caravan/trailer - which creates a terrible drag / suction force.
Minimise any protrusions that disturb the smooth flow of air (laminar flow) around any surface of the trailer. This includes lights, accessories such as power points, door handles, chassis structures underneath... even the rotating wheels themselves.
The minimisation of the frontal area had already determined that a pop-top caravan/camper trailer option was needed that ideally had a roof height of less than the roof height of the tow vehicle. With a number of other factors considered, we ended up with a Wayfinder 12F, but this still had room for improvement with overall aerodynamic design.
First up was the trailing edge. Replacing the square rear edge with a tapered "wedge" boot seemed the easiest solution. Not only would this help reduce trailing edge drag, but would have the two bonuses of increasing storage area (which caravan/campers are usually always short of) as well as increasing roof area, which means more area to fit more solar panels!
The design of the boot needed to account for not only the optimal angles of departure (if your angles are too sharp (> ~15 degrees), the airflow wont "stick" to the surface and will break away anyway, defeating the purpose and creating a suction void you are trying to avoid), but also need to comply with the limitations imposed by the Australian Design Regulations for trailer designs (e.g. maximum lengths/distances from axels, location of lights, licence plate and reflectors etc). A fair bit of play with CAD models and re-checking rules etc got me something I think will be acceptable to both requirements.
Original shape/cross section in respect to airflow:
Wedge boot with ~15 degree angle of departure, at near max allowable length of 3.7m from axle to lights!
with lid added...
Aluminium rails added for bracing and mounting of solar panel....
Showing its new operation with the Jacka electric roof lifter.
Second up was what could be done to improve the airflow at the front of the camper.
The easiest airflow to work with is the joining of the flat roof surfaces.
For this I created a hinged roof panel that could be adjusted in height to match different tow vehicle roof heights - and also enable testing at slightly higher or lower angles to find the most efficient angle. This also served to assist with the ADR requirements of extensions to the rear of the axle, had to be less than the distance of the loadable area forwards of the axle - plus the bonus again of more roof area for more solar panel capture area.
To completely eliminate the majority of any second "front" - I ideally plan to experiment with stretch canvas to join the car to the camper, creating a single aerofoil shape body - rather than 2 separate vehicles.
What it looks like, after painting the black stripes (but not the grey yet!)
The next step remaining is to smooth out the underbelly surface with an under tray - and wheel spats/covers!
Stay tuned!
Tim
What solar panels are you using, and why?
Hi Tim, Very informative. Excellent progress. Well done. I have M3 (lower roof height) so the adjustable front of trailer deflector is an excellent mod. Also, is that why teardrop trailers have the drop-down design that they have at the rear? Thanks for the post.